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Wire sizes?

Discussion in 'Model S: Battery & Charging' started by billarnett, Mar 7, 2015.

  1. billarnett

    billarnett Member

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    If I'm reading it right the main conductors in the cable from the HPWC to the car are 6 AWG. If that's right and code-compliant then why is it necessary to use #3 from the breaker box to the HPWC?

    Charging my car at 80A this evening I note that the HPWC cable is getting quite warm, about 100F according to my cheap IR thermometer. So too is the (non-compliant soon to be replaced) breaker.
     
  2. GSP

    GSP Member

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    The UMC uses two smaller conductors in parallel for each circuit. This makes the cable more flexible and maybe less costly.

    Perhaps the HPWC does the same, with two #6 AWG conductors?

    In any case, I would only charge at 80 A when I needed it, and charge at 56 A whenever that was enough to finish charging before I leave home. One of the smart guys on this forum suggested 56 A to cut the I^2R heat loss in half, but to still excersize the twin chargers. Why run the full heat load on your pannel, HPWC, connector, and car, when it is not needed? It will just wear things out faster.

    GSP
     
  3. billarnett

    billarnett Member

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    Yes, that's exactly what I intend to do. But I'm curious about the size of the HPWC conductors. There does appear to be three other conductors in the cable but they're much smaller and I assume for signaling not power.
     
  4. wycolo

    wycolo Active Member

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    > suggested 56 A to cut the I^2R heat loss in half [GSP]

    For the same job the total heat lost will be roughly the same but the heating effect will be less resulting in cooler wires & connections over a longer duration. [ I believe that's what that guy said ].
    --
     
  5. Bratwurst

    Bratwurst Member

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    The wire size you need to use from the breaker box to the HPWC is comp!etly dependent upon distance. There is a calculation for amp load x wire size x loss = diameter of conducter. The closer you are to the breaker box, the smaller wire you can use.

    100F is not out of range for a breaker or cord.
     
  6. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I'm the 70% guy. 80A * 70% = 56A

    Reducing current to 70% of the original means the Ohmic Power dissipated is about 50% of the original (0.7*0.7=0.49).

    When you do this, the total energy lost to the resistive components for the same total charge is about 30% less because the 50% power is used for 40% longer. You do save total energy and do run everything cooler.

    It's all designed for full power, so use that when you need it, but if you don't (for me, that is 90% of hone charges), then throttling back the charging can reduce some heat load everywhere.
     
  7. davewill

    davewill Member

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    The wiring to the HPWC is subject to the 80% rule and has to be sized for a full 100 amps. The wire in the HPWC only has to be sized for the 80a that the EVSE is actually rated for. I know it sounds faintly stupid, but...
     
  8. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    Incorrect. NEC specifies minimum wire size. The minimum wire size for supporting an 80A charging load (100A circuit size due to the continuous load rule) is a #3 AWG.

    Yes, the cable from the HPWC to the car uses #6 conductors rated at 105 degC, not in parallel. They do not fall under NEC jurisdiction - they fall under UL listing guidelines and are tested.

    One of the reasons the HPWC can use this is that HPWC cable is generally open-dissipation while infrastructure cabling happens in conduits, walls, and other cavities that can heat up.
     

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